Automatic volume control



AUTOMAT I C VOLUME CONTROL k x Y mm v vINVENTOR 2 CHRISTOPHER J. FRANKSQL BY f Y a Q l I l M ATTORNEY c. J. FRANKS4 AUTOMATIC VOLUME CONTROLMarch 22, 1938.

Filed May 14, 1955 2 Sheets-Sheet 2- iam/ww 0n/77090 W907- CH RISTOPHEBJ. FRANKS BY /fl Ww ATTORNEY Patented Mar. 22, 1938 UNITEDA STATESAUTOMATIC' VOLUME CONTROL Christopher J Franks,

Boonton, N. J., assigner to Radio Corporation of America, a corporationof Delaware A Application May 14,

8 Claims.

My present invention relates to superheterodyne receivers, and moreparticularly to novel and improved methods of, and devices for,automacally regulating the volume of superheterodyne 5 receivers.

It has heretofore been proposed to provide automatic volume controlarrangements for superheterodyne receivers. In such volume controlarrangements the gain of the converter netl0 Work has been varied, aswell as the gain of the various super-audible frequency amplifiers, inresponse to received signal carrier amplitude variations. 'For example,and as shown in my Patent No. 2,078,072, dated April 20, 1937, it hasbeen proposed to vary the gain of a pentagrid converter tube of asuperheterodyne receiver by varying the signal grid bias of theconverter tube,1this being accomplished in addition to the variation ofsignal grid bias of the radio frequency 2O and intermediatefrequencyamplifier stages.

However there are many situations wherein it is desired to have a morerapid automatic gain control action on the pentagrid converter tube of asuperheterodyne receiver. By increasing the rapidity of the automaticvolume control action on the converter tube it may be possible, forexjample, to dispense with automatic bias control of the super-audiblefrequency amplifier stages. One of the problems to be overcome inproviding a more rapid automatic gain control of the converter networkof a superheterodyne receiver involves the need for maintainingsubstantial local oscillator voltage in the converter network throughoutthe automatic gain control action.I y

Accordingly it may be stated Vthat it is one of the primary objects ofmy present invention Vto provide in a superheterodyne receiver employinga pentagrid converter network, an automatic volume control arrangementwhich functions 40 simultaneously to vary the converter tube gain andthe local oscillator voltage amplitude in response to signal carrieramplitude'rvariations.

Another important object of theinventionvis to provide in conjunctionwith a pentagrid converter network of a `superheterodyne receiver, anautomatic gain control arrangement which op- -erates'to vary the gain ofthe converter tubeby varying the negative grid biasr on the signal gridof the converter, 'and simultaneously varies the local oscillatorVvoltage amplitude in a sense suoli` that the oscillator voltageVamplitudeldecreases `appreciablywith signal amplitude in.- creasethereby providing amore rapid automatic `regulationof the converternetwork.'r z .Y

^Another:object of; the; ,inventionis to` provide 1935, serial No.21,341 (C1. asoao'y an improved and rapid automatic volume controlmethod for a superheterodyne receiver employing a pentagrid converternetwork, the con'- verter being particularly characterized by itsinclusion of a double feed back arrangement from 5 the oscillatoranodeelectrode and plate of the converter tube, andthe automatic volumecontrol arrangement functioning toincrease the negative bias on thesignal grid ofthe converter tube and thereby substantially renderyinoperative the feed l0 back from the plate of the converter tube asthe received signal carrier amplitude increases.

Still other objects ofthe present invention are to improve generallytheefficiency of superheterodynexreceivers employing pentagrid converters15 and utilizing automatic volume control, and more especially toprovidesuch receivers which are not only. reliable andeiiicient in operation,but economicallymanufactured and assembled.

The'novel features which I .believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself,.however, as to both its organization and method of operationwill best be understood by reference to thefoll'owing description takenin 25 connection with the drawings in which I have indicateddiagrammatically several circuit organizationswhereby my invention maybe carried into effect.

Y v.In the drawingsr- 1 Y 30 Fig. 1 diagrammatically shows asuperheterodyne` receiver embodying the present invention,

Fig'. 2 showsl a converter network of Fig. 1 embodying a modified formof the invention, Fig; Bshows a furtherfmodification of the con- 35verter network, :.L I f Y Fig. 4 graphically illustrates the operationof. athepresent invention.V Y

Referring now tothe accompanying drawings, -wherein like Areferencecharacters in the different 40 lgures designatesimilar circuit elements,there Y isshownzin Fig. 1 thenetworks of a superheteroydyne receiver. ofconventional and well known construction; The receiver embodies a sourceof signals l, and thismay comprise the usual signal collector lwhich maybe a grounded antennacircuit; a loop antenna; Yan automobile signalpickup device and even aradio frequency distribution line such asusedinY hotels or apartmenthouses zatthepresent time. The source I may,also, be

considered :as including one or more, stages of tunable radio frequencyamplification, and the number of stages to be employed will depend .upon,the signal amplitude 4desired at the input circuit ofthe pentagridconverter tube. l

The signal source I is followed by a pentagrid converter tube 2 which isof the well known 6A'7 type. Since the electrode structure of such atube is well known at the present time, and its circuits, and theirfunctions, are also very well known, it is sufficient to point out thatthe converter functions to convert the signal input energy to a desiredintermediate frequency which is produced in the intermediate frequencyoutput circuit 3 of the converter tube.

The signal energy is impressed upon the tunable signal input circuit 4of converter tube 2, and the tunable local oscillator network 5functions to tune the local oscillator network to that frequency whichwill differ from the frequency of input circuit 4 by the frequency ofthe network 3. The converter tube is followed by an intermediatefrequency amplierfnetwork 6; the latter may comprise one, or more,stages of intermediate frequency amplification. The network 3-3 isresonant tothe operating intermediate frequency, and this frequency maybe chosen from a range of 75 to 450 kc. The amplified output of theintermediate frequency amplifier network (i is then impressed upon asecond detector 1, and the latter may be of any desired type. Thedemodulated output of the second detector 'I is impressed upon an audionetwork, and the latter may comprise one, or more, stages of audiofrequency amplification, followed by a reproducer.

In order to overcome the effects of variations of received signalstrength there is employed an automatic volume control arrangement, andthis latter arrangement functions to maintain the signal carrieramplitude at the input of the demodulator 'l substantially constant overa wide range of signal carrier amplitude variation at the signalcollector of the receiver. The automatic volume control arrangement(hereinafter designated as AVC) may comprise a rectifier 8 of anydesired and well known type, and the function of the rectifier is toproduce a` Varying direct current voltage across the output loadresistor 9. The varying direct current voltage is impressed as a gaincontrol bias upon the signal grid of the converter tube 2, and this isaccomplished through the lead I which includes the filter network II.

The function of the network Il is to suppress pulsating components ofthe rectified signal energy, and prevent them from being impressed onthe signal grid of the converter tube 2. Of course, there may beadditional variable bias leads from lead I0 to the intermediateVfrequency amplifier grid circuits, and also to the grid circuits of theradiofrequency amplifier. However, such additional leads are .not shownsince such control circuits are well known to those skilled in the art.While the demodulator 1. and rectifier 3 are shown as conventional innature, it will be understood that any desired type of specific circuitsmay be used for accomplishing their functions. For example, themulti-function tubes disclosed in my aforesaid copending application maybe utilized for these demodulation and rectification functions. Ingeneral, it is to be understood that any of the automatic volume controlcircuits shown in my aforesaid copending application may be utilized inconjunction with the tunable converter tube 2 disclosed herein.

The pentagrid convertertube 2 Vincludes the usual cathode and plate, andthe intermediate five grids. The grid Grfunctions as the localoscillator grid, while the grid G2 functions as the oscillator anode.Grid G4 has the signal energy impressed thereon, and the signal grid isdisposed between a pair of screen grids which are connected to a pointof positive direct current potential, the screening grids functioning aselectrostatic screens because they are at ground alternating currentpotential, The cathode of converter tube 2 is connected to groundthrough the usual signal grid bias resistor I2, the latter beingsuitably bypassed by condenser I3. The variable tuning condenser E4 inthe signal input circuit 4 has the grounded side thereof connected tothe low alternating potential side of the signal input coil through ablocking condenser I 5.

The variable tuning condenser I of the local oscillator network 5 hasone side thereof grounded, while its high potential side is connected tothe oscillator grid G1 through a condenser Il. The grid side ofcondenser I'.' is connected through the resistor I8 to the cathode. Theresistor I8 and condenserl'l function as a leaky grid condenser networkfor the local oscillator section of the converter network. The dottedline representation used in conjunction with condensers I4 and I6designates that these two condensers have their rotors mechanicallyuni-controlled, and it is to be understood that the local oscillatorcircuit 5 also is provided with proper padding condensers to keep thelocal oscillator network 5 properly tracked with the tuning of thesignal input circuit 4.

Whereas in prior pentagrid converter arrangements it has been thepractice to provide feedback from the oscillator anode G2 to theoscillator grid G1 in order to create the local oscillations, in thepresent converter network the plate of the converter tube 2 is alsoutilized to provide feedback. This double feedback arrangement isprovided in order to have a strong local oscillator Voltage amplitudewhen weak signals are received, and a substantially weak oscillatorvoltage amplitude when relatively strong signals are received. This isaccomplished by connecting the plate of converter tube 2 to anappropriate source of positive potential B through a path which includesthe coil of the intermediate frequency output network 3 and theoscillator feedback coil 20.

The coil 2U is magnetically coupled to the coil 2l of the localoscillator network 5. denser 22 connects the B side of feedback coll 2Uto ground for proper bypassing action. The oscillator anode G2 isconnected to a source of positive potential L, which potential has amag- 'nitude substantially less than that of source B,

through a path which includes the radio frequency choke 23, the lowpotential side of coil 23 being connected to ground through a properbypassing condenser 24. The plate side of feedback coil 20 is connectedto the grid side of choke 23 through a blockingcondenser 25, and thelatter is given a magnitude of about 250 micromicrofarads.

It will, therefore, be seen that the grid G2 feeds back radio frequencyenergy to the oscillator grid G1 through a path including condenser 25,the feedback coil 2U and the oscillator circuit 5. The plate ofconverter tube 2, also, feeds back radio frequency energy to theoscillator grid G1 through a path including feedback coil. 20 andoscillator circuit 5. By means of this arrangement the grid G2 does notfurnish the entire oscillating anode conductance. 'I'he Vplate of theconverter tube furnishes some of this oscillating The conanodeconductance, andthe latter is controlled substantially vto cut-off bythe increasing negative bias on the signal grid G4. Ultimately, andafter the point of cut-off of the plate feedback through coil 20, therewill only be left the feedback action due tothe grid G2, and thisfeedback action is designed to be sufficient to keep the oscillatorvoltage of sufficiently high andi adequate amplitude no matter whatvalue of AVC bias is applied to G4.

The arrangement shown in Fig. 1 is the preferred embodiment of theinvention, Whereas Figs. 2 and 3 show alternative embodiments whichfromV practical considerations are not as preferable as the arrangementshown in Fig. 1. In Fig. 2, for example, the oscillator Yanode G2 isconnected by a lead to a point on the feedback coil 20, this point beingatV a lower radio frequency potential than the point of coil 20 to whichthe plate of tube2 is connected. In other words, in Fig. 2 there isshown an alternative embodiment for securing the result secured with thearrangementlshown in Fig. 1 by tapping the grid G2 down upon thefeedback coil 20.

It will be observed that the arrangement in Fig. l differs from thatshown in Fig. 2 in that while the grid G2 is connected to the same pointof feedback coil 20 as the plate of tube 2 inso far as radio frequencypotentials are concerned, yet it is provided with direct current voltagethrough a parallel path and through the choke 23. The energizing directcurrent voltage L applied to grid G2 in Fig. l is substantially lowerthan that applied by source B to the plate so that the initial feedbackdue to the grid G2 is much less than that from the plate.v In Fig. 2

this diminished initial feedback bythe grid G2IV is secured by tappingVdown the grid upon the feedback coil 20, and supplying the plate andgrid G2 from a common source of voltage B.

The alternative embodiment in Fig. 3 resembles that shown in Fig. l inthat thevoltageL is applied to the coil G2, but an auxiliary feedbackcoil 20' is utilized. The auxiliary feedbackcoil 20', connected inI thelead to the gridV G2, is Wound on the plate feedback coil 20. The coils20 and 20 are poled so that the voltages which they induce into circuit5 are in phase. They are Wound in the same direction,` and like ends aregrounded. 'Ihis'modif'lcatiom of course, differs from that shown in Fig.2 in that the energizingdirect current voltage ofthe grid'G2 is muchlower than that used for the plate of the converter tube. As statedheretofore, the arrangement shown in Fig. 1 is preferable to those shownin Figs. 2 and 3. From practical considerations, it will be observedthat in the arrangement of Fig. 1 there is avoided the necessity ofcutting down the plate turns on the oscillator feedback coil, and thereis, furthermore, avoided the need' for an auxiliary feedback coil.However, it is to be clearly understoodthat the operation andfunctioning of the circuits'are substantially the same, the circuitarrangement in Fig. 1 being more efficient insofar as the rapidity ofvolume control action is concerned.

Considering now the operation of the invention, and with particularreference to the circuit arrangement of Fig. 1, it is pointed out thatwhen relatively Weak signals are received the AVC bias on the signalgrid G4 is substantially zero, and the converter tube operates at itsmaximum gain. Both feedback paths are then functioning at `their maximumefficiency. As pointed out heretofore theV energizing voltage L on thegrid G2 is such thatthe initial feedback from this grid is much lessthan that from the plate of the converter tube. In Fig. 4 there isgraphically shown these relationships. CurvevA denotes the variation ofoscillation voltage amplitude with increasing negative bias on grid G4with feedback from rthe plate of converter tube 2 alone. The curve Bshows the same relationship for the grid G2 furnishing feedback aloneand with substantially |32 Volts on the gridGz.

By way of contrast the curve C shows the same relationship for the gridG2 feeding back by itself with +15 volts on this grid. The curves D andD show the resultant oscillator amplitude variation curves.- In otherwords these curves show the change in oscillator amplitude, consideringthe effects of the plate feedback and the grid G2 feedback. It will beobserved from the curves of Fig. 4 that with zero AVC bias on grid G4there is a maximum resultant oscillator voltage amplitude produced inthe converter tube 2, and that the component of oscillator voltage dueto the grid G2 is substantially less than that dueto the plate acting byitself. The curve D shows the resultant oscillator curve due to curves Aand C; curve D shows the resultant of the curves A and B.

As the signal carrier amplitude increases, the negative bias on signalgrid G4 increases; and eventually cuts oif the feedback from the plateof the converter tube. Considering Fig. 4 again, it Will be seen fromcurves D and D' that When the signals have reached the point wheresubstantial AVC bias is produced the effective oscillator voltageamplitude has been substantially reduced to the point where curves B andC correspond With the resultant oscillator voltage amplitude. f

Thus there is a substantial decrease of the effective oscillatoramplitude from zero AVC bias to large values of such bias. As a resultof the present invention there is not only secured a reduction in gainof the converter tube due to the increasing bias of signal grid G4, butthe local oscillator voltage amplitude is substantially decreased, andboth these effects combine to secure a very rapid automatic volumecontrol action.

It is to be particularly noted that the decrease of the effectiveoscillator voltage: amplitude, even at large AVC bias values, is notsuicient to interfere with the converter action of tube 2. The gridA G2,as shown by curves B and C in Fig. 4, feeds back sufficient radioVfrequency energy to produce a satisfactory and adequate oscillatorvoltage amplitude.V

By way of example, and in no sense limiting, it is pointed out that ingeneral the magnitude of the B voltage is about three times as great asVthe voltage L. Thus, Where L is about 32 volts, the value of B is some100 volts. If 250 volts were used on, the. plate,.as is normal for 6A7operation, then the value of L should .be in the range of 75 to 150volts.

While -I have indicated and described several systems for carrying myinvention .into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

l. In a converter network using a pentagrid tube provided withoscillator and mixer electrodes, the method of automatically controllingthe output of the converter which includes feeding back radio frequencyenergy from the oscillator anode, additionally feeding back energy fromthe mixer plate of the converter tube to an extent substantially greaterthan the oscillator feedback, and increasing the negative bias on thesignal grid of the converter tube as received signal carrier ampltiudeincreases whereby the feedback from the aforesaid plate is substantiallyeliminated at a predetermined value of negative bias on the signal grid.

2. In a converter network using a pentagrid tube provided withoscillator and mixer electrodes, the method of automatically controllingthe output of the converter which includes feeding back radio frequencyenergy from the oscillator anode, additionally feeding back energy fromthe mixer plate of the converter tube, increasing the negative -bias onthe signal grid of the converter tube as received signal carrieramplitude increases whereby the feedback from the aforesaid plate issubstantially eliminated at a predetermined value of negative bias onthe signal grid, and maintaining the initial feedback from theoscillator anode much less than that from the plate.

3. In a detector-oscillator system using a tube having oscillatorelectrodes and detector electrodes, the method of automaticallyregulating the gain of the tube in response to received signalvariations consisting in feeding back radio frequency energy from thedetector output electrode to the oscillator grid electrode,proportioning the last feedback and the oscillator normal feedback sothat the last feedback initially predominates, and substantiallyeliminating the last feedback when signals above a desired amplitude arereceived.

4. In a detector-oscillator system using a tube having oscillatorelectrodes and detector electrodes, the method of automaticallyregulating the gain of the tube in response to received signalvariations consisting in feeding back radio frequency energy from thedetector output electrode to the oscillator grid electrode,proportioning the last feedback and the oscillator normal feedback sothat the last feedback initially predominates, substantially eliminatingthe last feedback when signals above a desired amplitude are received,and maintaining the normal oscillator feedback of adequate amplituderegardless of the amplitude of received signals.

5. A converter network including a pentagrid tube provided withoscillator and mixer electrode sections, means for providing a doublefeedback from the oscillator anode electrode and the mixer outputelectrode, the mixer output electrode feedback initially exceeding theoscillator anode feedback to a substantial extent, and automatic gaincontrol means for increasing the negative bias on the tube signal grid,as signals increase, to thereby eliminate the mixer feedback.

6. In combination, a tube having at least a cathode, an oscillator grid,an oscillator anode electrode, a signal grid and an output electrode, asignal input circuit connected between the cathode and signal grid, anetwork including a resonant circuit tuned to a predetermined localoscillation frequency, coupling the oscillator anode and grid, a beatfrequency circuit connected to said output electrode, means reactivelycoupling the output electrode and the oscillator grid to provide anenergy feedback and thereby produce oscillations of said local frequencywhich are of an amplitude substantially exceeding the amplitude ofoscillations produced by said first coupling, said signal grid beingdisposed between the oscillator anode and output electrode, and meansfor varying the direct current potential relations between the signalgrid and cathode thereby to regulate the magnitude of energy feedbackthrough said reactive coupling means.

'7. In combination, a tube having at least a cathode, an oscillatorgrid, an oscillator anode electrode, a signal grid and an outputelectrode, a signal input circuit connected between the cathode andsignal grid, a network including a resonant circuit tuned to apredetermined local oscillation frequency, coupling the oscillatoranthereby to control the energy feedback through "l said reactivecoupling means, said output electrode being maintained at asubstantially greater positive direct current potential than saidoscillator anode whereby the feedback from said oscillator anode is aminimum when said potential difference is a minimum.

8. In combination, a tube having at least a cathode, an oscillator grid,an oscillator anode electrode, a signal grid and an output electrode, asignal input circuit connected between the cathode and signal grid, anetwork including a resonant circuit tuned to a predetermined localoscillation frequency, coupling the oscillator anode and grid, a beatfrequency circuit connected to said output electrode, means reactivelycoupling the output electrode and the oscillator grid to produce afeedback of energy which substantially exceeds the energy feedback dueto said first coupling, said signal grid being disposed between theoscillator anode and output electrode, and means for varying the directcurrent potential relations between the signal grid and cathode therebyto control the energy feedback through said reactive coupling means,said last varying means comprising a beat frequency energy rectifier,and connections for impressing between the cathode and signal grid thedirect current output of the rectifier.

CHRISTOPHER J. FRANKS.

